Current repair processes for integrated circuit (IC) chips and for lithographic exposure reticles (i.e. masks) generally rely on formation of highly localized areas of energy on the surfaces thereof, to induce spatially confined endothermic reactions for selective etching or deposition of materials. Focused beams of ions, electrons or photons are used to form these localized areas. Focused ion beams (FIBs) have been shown to have superior process confinement and reaction rates compared to focused electron beams (e.g. in SEMs) or laser beams. FIB tools have thus assumed a dominant role in most repair applications, as well as for site-specific cross-sectioning in the failure analysis of ICs.
Serious difficulties in chip and mask repair have recently been encountered due to the introduction of new materials and structures that are not compatible with conventional FIB processes. In particular, chips with copper metallization and/or polymeric low-k dielectrics present challenges in the selective removal of metal in back-end-of-the-line processing (“BEOL chip edit”). FIG. 1 illustrates a typical edit process where a FIB 10 is directed into an opening 1 exposing a metal line 12 in a substrate 2. The FIB milling of the copper forms conductive byproducts having low volatility, so that deposits 13 of conductive material are formed on the walls of the opening 1. Scattering of the ion beam also causes damage to the interlevel dielectric 2 in areas 14 along the wall. Polymer-based organic interlevel dielectrics may even become conductive when exposed to the ion beam.
In addition, imaging the chip surface with FIB or SEM (electron beams) during navigation to the repair site can result in damage to the interlevel dielectric material. Similarly, exposure to an ion beam in a mask repair process may lead to undesired changes in the optical index of the mask features (an effect often referred to as “staining”).
Besides materials-related issues, the constantly increasing packing density of IC features (that is, reduction in the minimum spacing between features) leads to serious problems in editing ICs and masks. The difficulty of editing a feature without disturbing its nearest neighbors rapidly increases as the spacing between features decreases. The minimum feature spacing is the major factor limiting the use of optical techniques for editing (e.g. laser beam based editing), due to the optical diffraction limit of spatial resolution for these techniques. In the case of ion and electron beam editing, the area of spatial confinement of the beam is much larger than the primary spot size. In general, the spatial resolution of beam-induced processing is an order of magnitude worse than the imaging resolution of the beam-generating apparatus (FIB or SEM).
Metrology for ICs presents additional challenges. In-line measurements are needed to provide accurate three-dimensional data for IC fabrication process development and control in the IC manufacturing line. The feature to be measured must be sectioned without changing the shape of the original structure before the profile measurement is performed. In a typical FIB- or SEM-based profile metrology process, ion- or electron-beam induced deposition is performed to encapsulate the structure to be sectioned. However, damage to the original surface may occur due to exposure to the ion or electron beam before an adequate thickness of a protective layer is deposited.
The arrangement of components inside typical FIB and SEM chambers makes it difficult to view the sample being processed. FIB and SEM chambers generally cannot accommodate a conventional optical microscope for directly viewing the sample, or for providing optical exposure to the region of the sample interacting with the beam. Two proposed solutions to this problem are as follows: (1) A Schwartzschild reflective microscope with an infrared (IR) objective lens, built into the ion beam forming column; this type of in-situ optical microscope requires a large physical working distance, resulting in a low numerical aperture (NA) and relatively poor image resolution. (2) An in-situ microscope column physically offset from the beam-to-sample interaction region, so that the sample must be shifted back and forth with a known offset.
It is desirable to obtain a magnified image of the sample, but space limitations inside the FIB/SEM chamber generally require that the working distance be very small. This in turn causes difficulties in illuminating the sample, and requires an approach different from conventional endoscopy. Endoscopes typically produce de-magnified images of macroscopic areas, such as the inner walls of a patient's colon or the interior of an engine cylinder. Since a large working distance (typically more than 5 mm) is used in conventional endoscopic devices, sample illumination can be provided using a light source and optical fibers positioned off-axis with respect to the main image collection fiber. However, to obtain magnification with the appropriate NA (that is, no false magnification), the working distance must be reduced to a value typically less than the diameter of the imaging lens. This means that any off-axis illumination scheme would result in physical shadowing by the imaging objective lens. Accordingly, lack of illumination of the sample region of interest has been one of the critical issues limiting the ability to produce magnified (e.g. >10×) images with an endoscope. In addition, magnification after image translation is not feasible due to the intrinsic resolution loss caused by fiber-based image guides. Resolution loss occurs in coherent fiber bundle image transmission because of the finite spacing achievable between pixel elements. In the case of single fiber-based gradient index (GRIN) image guides, which behave as a train of convex lenses continually re-focusing light toward the optical axis, resolution is lost due to the intrinsically low NA.
There is a need for an apparatus and technique (i) for preventing or limiting damage caused by ion or electron beam repair or metrology processes; (ii) for navigating to a feature of interest on the surface of the chip or mask without subjecting a large surface area to beam damage; and (iii) for conveniently viewing the sample in the process chamber.